It is standard to heat a machine part, such as an injection-molding nozzle, by fitting it with an electric sleeve heater that is engaged snugly around the part and energized so as to keep the part at a predetermined temperature.
Such a sleeve heater comprises a resistance-type heater coil whose inside diameter is slightly greater than that of the part over which it is to be fitted. Frequently heat-distributing material is provided inside the coil. In order to tighten the coil, it is provided with an external clamping arrangement that urges its inner surface into good heat-transmitting contact with the part to be heated. Such a clamping arrangement can include an outer sleeve provided with a wedge-type screw-operated tightening system that reduces the coil's diameter. Such an arrangement makes the heater fairly bulky so it is difficult to fit in a restricted space, and substantially complicates the installation and use of the sleeve heater. The ends of the coil are attached to the sleeve, further complicating tightening of the coil on the element being heated.
Such heaters may also be installed by externally mounting them on a cylindrical part, for example a nozzle for an injection-molding machine, so that the melt flowing through is heated by the heating cartridge. However, it is also possible to heat a tube around which a melt flows, by inserting the helical electric heating cartridge into the tube and pressing against the tube surface in order to heat the melt or the like externally flowing past the tube.
It is furthermore known to provide a screw-type tangential tightener which is somewhat more compact, but which nonetheless still is difficult to use with closely spaced nozzles. When the part being heated is of an exactly determined size, it is known to slip over the coil an outer sleeve that is dimensioned to compress it to the exact inner diameter desired, and then solder it in place before fitting the coil over the part. This latter arrangement is problematic in that any variation in size makes the heater impossible to install or so loose when installed as to be ineffective.
In another known arrangement described in U.S. Pat. No. 7,049,555 the sleeve heater has an electrical and generally cylindrical heater coil centered on an axis and shaped to fit over a part to be heated, a radially compressible and generally cylindrical inner sleeve snugly coaxially externally surrounding the heater coil, radially inwardly bearing on the coil, and having an axially outwardly projecting tab, and a radially generally inextensible and generally cylindrical outer sleeve fitted coaxially over the inner sleeve and having an inner surface bearing tightly radially inward on the inner sleeve and radially compressing the inner sleeve and the coil inward. Once again, the ends of the coil are fixed to the sleeve so that the coil itself must be radially compressed, which can damage the coil.